Use placeholder for search action

Summary:
See https://hig.kde.org/style/writing/placeholder.html

Screenshot:

Old: {F6691712}
New: {F6698685}

Test Plan: Compile and run

Reviewers: #dolphin, #vdg, ngraham, elvisangelaccio

Reviewed By: #dolphin, #vdg, ngraham

Subscribers: GB_2, kde-doc-english, elvisangelaccio, ngraham, yurchor, kfm-devel

Tags: #dolphin, #documentation

Maniphest Tasks: T10258

Differential Revision: https://phabricator.kde.org/D19770